Honor CONFIG_FS_POSIX_ACL kernel option

The required Posix ACL interfaces are only available for kernels
with CONFIG_FS_POSIX_ACL defined.  Therefore, only enable Posix
ACL support for these kernels.  All major distribution kernels
enable CONFIG_FS_POSIX_ACL by default.

If your kernel does not support Posix ACLs the following warning
will be printed at ZFS module load time.

  "ZFS: Posix ACLs disabled by kernel"

Signed-off-by: Massimo Maggi <me@massimo-maggi.eu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1825

include/linux/vfs_compat.h

@@ -174,6 +174,7 @@ lseek_execute(struct file *filp, struct inode *inode,
 }
 #endif /* SEEK_HOLE && SEEK_DATA && !HAVE_LSEEK_EXECUTE */
 
+#if defined(CONFIG_FS_POSIX_ACL)
 /*
  * These functions safely approximates the behavior of posix_acl_release()
  * which cannot be used because it calls the GPL-only symbol kfree_rcu().
@@ -315,5 +316,6 @@ typedef umode_t zpl_equivmode_t;
 #else
 typedef mode_t zpl_equivmode_t;
 #endif /* HAVE_POSIX_ACL_EQUIV_MODE_UMODE_T */
+#endif /* CONFIG_FS_POSIX_ACL */
 
 #endif /* _ZFS_VFS_H */